This description of OWL, the Web Ontology Language
being designed by the W3C Web Ontology Working Group,
contains a high-level abstract syntax for both OWL DL and OWL Lite,
sublanguages of OWL.
A model-theoretic semantics is given to provide a formal meaning for OWL
ontologies written in this abstract syntax.
A model-theoretic semantics in the form of an extension to the RDF model
theory is also given to provide a formal meaning for OWL ontologies
as RDF graphs (OWL Full).
A mapping from the abstract syntax to RDF graphs is given and
the two model theories are shown to have the same consequences on
OWL ontologies that can be written in the abstract syntax.

This is
a W3C
Web Ontology Working Group
Working Draft produced
3 February 2003
as part of the W3C
Semantic Web Activity
(Activity Statement).
It incorporates decisions made by the Working
Group in designing the OWL Web Ontology Language.
This is
a public W3C Working Draft and may be updated, replaced, or
obsoleted by other documents at any time.
However, it is expected that this working draft is quite close to the Last
Call version of the document.
It is inappropriate to use W3C
Working Drafts as reference material or to cite as other than "work in
progress". A list of current W3C Recommendations and other technical
documents can be found at
http://www.w3.org/TR/.

1. Introduction

This document contains several interrelated specifications of the several
styles of OWL, the Web Ontology Language being produced by the
W3C Web Ontology Working Group
(WebOnt).
First, Section 2 contains
a high-level, abstract syntax for both
OWL Lite, a subset of OWL,
and OWL DL, a fuller style of using OWL
but one that still places some
limitations on how OWL ontologies are constructed.
Eliminating these limitations results in the full OWL language, called
OWL Full, which has the same syntax
as RDF.
The normative exchange syntax for OWL is
RDF/XML [RDF Syntax];
the OWL Reference document
[OWL Reference]
shows how the RDF syntax is used in OWL.
A mapping from the OWL abstract syntax to
RDF graphs
[RDF Concepts]
is, however, provided in Section 4.

This document contains two formal semantics for OWL.
One of these semantics, defined in
Section 3,
is a direct, standard model-theoretic semantics for
OWL ontologies written in the abstract syntax.
The other, defined in Section 5,
is a vocabulary extension of the RDF model-theoretic semantics
[RDF MT] that provides semantics
for OWL ontologies in the form of RDF graphs.
Two versions of this second semantics are provided, one that corresponds
more closely to the direct semantics (and is thus a semantics for OWL DL)
and one that can be used in cases where classes need to be treated as
individuals or other situations that cannot be handled in the abstract
syntax (and is thus a semantics for OWL Full). These two versions are
actually very close, only differing in how they divide up the domain of
discourse.

Appendix A
contains a proof that the direct and RDFS-compatible semantics have the same
consequences on OWL ontologies that correspond to abstract OWL
ontologies that separate OWL individuals, OWL classes, OWL properties,
and the RDF, RDFS, and OWL structural vocabulary.
For such OWL ontologies
the direct model theory is authoritative and the RDFS-compatible model
theory is secondary.
Appendix A
also contains the sketch of a proof that the entailments in the
RDFS-compatible semantics for OWL Full include all the entailments in
the RDFS-compatible semantics for OWL DL.
Finally a few examples of the various concepts defined in the document are
presented in Appendix B.

This document is designed to be read by those interested in the
technical details of OWL. It is not particularly intended for the
casual reader, who should probably first read the OWL Guide
[OWL Guide]. Developers of parsers
and other syntactic tools for
OWL will be particularly interested in Sections
2 and 4.
Developers of reasoners and other semantic tools for OWL will be
particulary interested in Sections
3 and 5.

1.1. Differences from DAML+OIL

The language described in this document is very close to the
DAML+OIL web ontology language
[DAML+OIL].
The only substantive changes between OWL and DAML+OIL are

the removal of qualified number restrictions;

the ability to directly state that properties can be symmetric;

a new construct for stating that several individuals are distinct;
and

the absence in the abstract syntax of some abnormal DAML+OIL constructs,
particularly restrictions with extra components.

There are also a number of minor differences between OWL and DAML+OIL,
including a number of changes to the names of the various constructs, as
mentioned in Appendix A of the
OWL Reference Description
[OWL Reference].

The following table provides pointers to information about each
element of the OWL vocabulary, as well as some elements of the RDF and RDFS
vocabularies.
The first column points to the vocabulary element's major definition
in the abstract syntax of Section 2.
The second column points to the vocabulary element's major definition
in the OWL Lite abstract syntax.
The third column points to the vocabularly element's major definition
in the direct semantics of Section 3.
The fourth column points to the major piece of the translation from
the abstract syntax to triples for the vocabulary element
Section 4.
The fifth column points to the vocabularly element's major definition
in the RDFS-compatible semantics of Section 5.